Recently resistance variable memory elements, which include Programmable Conductive Random Access Memory (PCRAM) elements, have been investigated for suitability as semi-volatile and non-volatile random access memory devices. A typical PCRAM device is disclosed in U.S. Pat. No. 6,348,365 to Moore and Gilton. In typical PCRAM devices, conductive material, such as silver, is incorporated into a chalcogenide material. The resistance of the chalcogenide material can be programmed to stable higher resistance and lower resistance states. The unprogrammed PCRAM device is normally in a high resistance state. A write operation programs the PCRAM device to a lower resistance state by applying a voltage potential across the chalcogenide material.
The programmed lower resistance state can remain intact for an indefinite period, typically ranging from hours to weeks, after the voltage potentials are removed. The PCRAM device can be returned to its higher resistance state by applying a reverse voltage potential of about the same order of magnitude as used to write the element to the lower resistance state. Again, the higher resistance state is maintained in a semi-volatile manner once the voltage potential is removed. In this way, such a device can function as a resistance variable memory element having two resistance states, which can define two logic states.
A PCRAM device can incorporate a chalcogenide glass comprising germanium selenide (GexSe100-x). The germanium selenide glass may also incorporate silver (Ag) or silver selenide (Ag2Se).
The amorphous nature of the chalcogenide glass material used in a PCRAM device has a direct bearing on its programming characteristics. Thus, incorporation of silver into the chalcogenide glass requires precise control of the glass composition and silver concentration so as not to cause the chalcogenide glass to change from the desired amorphous state to a crystalline state.
Accordingly, there is a need for a resistance variable memory element having improved memory retention and switching characteristics.